The present invention relates to X-ray diagnostic imaging systems and more particularly to the systems' regulation of the X-ray exposure.
Conventional X-ray imaging equipment has a vacuum tube which when electrically excited emits X-rays. This tube includes a filament to heat the cathode of the tube to an operating temperature. Once at this temperature, a high d.c. voltage is applied across an anode and a cathode resulting in an electron beam bombarding the anode to produce X-ray emission. The X-ray tube can be electrically modeled as a variable resistor, the resistance of which being a function of the temperature of the tube's filament, and therefore the filament current. Within the filament's operating temperature range, the emission current flowing between the anode and the cathode, and hence the X-ray emission, is proportional to the filament current.
Because of the hazards associated with overexposure to X-rays, as well as the need to control the exposure for accurate imaging purposes, it is necessary to closely regulate the X-ray emission. One previous method of accomplishing this regulation involved continuously comparing the actual anode-to-cathode voltage to a reference level and varying the filament current, based on the result of the comparison, until the desired voltage was achieved. The feedback loop contained an amplifier, in the filament current supply, having its gain controlled by the voltage comparison. In an improved version of this method, the filament current also was sensed continuously to produce a feedback signal which controlled the amount of current applied to the filament.
This type of exposure regulation suffers from the relatively long thermal time constant of the filament which prevents rapid control of the high voltage across the anode and cathode of the tube. Furthermore, as each X-ray tube has slightly different characteristics, one cannot accurately provide a fixed compensation for variation of the anode-to-cathode excitation voltage. In addition, some X-ray exposures may be so short in duration that the exposure does not last beyond the initial period when conventional regulation is inaccurate.